In different kinds of manufacturing and production processes there are nips between rollers. A nip may be formed when two rollers or cylinders are pressed against each other and is defined by the width of the nip and the pressure profile. A nip may also be formed when a substrate is received between two rollers, where the rollers have a gap between each other without any substrate there between. The nip is either performing a transportation function or a modification function of a substrate or fluid. Nips may occur separate or in several steps in a process line. The nip is often formed of one soft roller and one hard roller pressed against each other or against a substrate. In some embodiments the nip is formed of two soft rollers or two hard rollers. As a soft roller normally has a coating formed of a rubber polymer, its characteristics, such as elasticity and hardness, will change over time, depending mainly on mechanical treatment, working temperature and influence of chemicals.
In order to over time guarantee the quality of the characteristics of the nip, it is desirable to measure important characteristic parameters, such as length of the nip and force applied in the nip, at an empirically established time interval. Such a measuring may be made by using a sensor in the form of a thin blade, which is placed in the nip between the rollers and which sensor then will measure either nip width or the pressure profile. For nips only formed if a substrate is placed between the two rollers, the sensor may be placed on a substrate dummy, simulating the normal thickness of the substrate. The measuring characteristics of the sensor must be monitored over time, in order for possible deviation to be noted or corrected. One example of such a roller nip gauge is given in WO 03/027623.
A sensor for length measurement may comprise two paths, one conductive path and one resistive path, each printed on a plastic substrate with a spacer sheet there between. A sensor for force measurement may comprise two plastic sheets with a printed measuring element placed there between.
By using a calibration unit the characteristics of the sensor may be monitored over time and possible changes may be documented in a quality system, or a calibration may be combined with an automatic adjustment of the changed characteristics of the sensor. Such an automatic adjustment can be achieved if the sensor during calibration at the same time is connected to its displaying measuring instrument.